1. Field of the Invention
The present invention relates to a diesel engine particulate filter (DPF) designed to physically trap particulate matter (PM) contained in diesel engine exhaust gases and particularly to a DPF in which heating and combustion of the PM are possible.
2. Description of the Related Art
The principal component in diesel engine exhaust gases is typically defined as solid type particulate matter (PM) made of inorganic carbon, also referred to as diesel soot.
Due to the acute and chronic effects on public health, carbon and other particulate substances should not be emitted into the atmosphere, thereby contributing to the level of man-made air pollution.
In view of the foregoing, a diesel engine particulate filter (DPF) integrated with the diesel engine's exhaust system is needed to trap emitted PM in the filter part and incinerate the particulates.
As an example of such a conventional DPF, Asakura Publishing Company, Ltd. printed a book dated Jul. 10, 1997 by the Society of Automotive Engineers of Japan, Inc., titled “Automobile Technical Series” (Volume 1), which contained an editorial “Automobile Motor Technology Corresponding to the Environment” on pages 139-148.
Another description was published by Sankaido Incorporated in their Jan. 10, 1994 issue, which contained an article written by Naoya Miyashita and Hideo Kuroki titled “The Diesel Engine for Cars” on pages 53-54.
Under normal operating conditions to burn PM trapped in the filter part of the DPF, it is necessary to heat PM to the reaction temperature of at least 550 degrees centigrade (1,022 degrees Fahrenheit), which is about the established spontaneous combustion temperature of PM.
For this reason, there are numerous conventional DPF which provide an electric heater to generate heat in the DPF.
In addition, there are other various adopted combustion systems. There are assorted DPF which burn carbon that is the principal component of PM at temperatures of 250 degrees centigrade or more. For instance, silicon dioxide, manganese oxide, and aluminum oxide powder mixed and sintered catalysts are used as an oxidation catalyst carried in the aluminum oxide coating with high dispersed platinum. These different methods facilitate regeneration (“burning off” process) and capture nitrogen dioxide (NO2) in the exhaust. This NO2 is used as a catalyst for PM combustion.
However, in the above-mentioned DPF using a conventional electric heater, while it is possible to ignite PM according to the condition of the filter part, a significant amount of electrical power is consumed to generate heat above 550 degrees centigrade. Furthermore, it is very difficult to continuously maintain the aforementioned temperature with the battery loading of usual vehicles.
Accordingly, although such an electric heater system is suitable for instance in a forklift which operates within the confines of a factory and the battery recharged while inactive from a 200V power supply on the premises, it is unsuitable for vehicles similar to a regular transportation truck outside the premises and not accessible to an external power source.
On the contrary, the above-mentioned DPF using the conventional NO2 as a catalyst, an electric heater is not necessary as it is possible to burn PM exhaust at the temperature of about 250-300 degrees centigrade or lower than using an electric heater. However, exhaust temperature will vary in the DPF during driving time. For example, exhaust temperature while driving in ordinary urban districts on average will reach 200 degrees centigrade or less; whereas, traveling on the highway slightly exceeds 250 degrees centigrade at least part of the time.
Consequently, even during short runs operating at high speed, driving conditions almost never reach the exhaust temperature needed to completely burn the trapped PM. Moreover, since NO2 is generated and used as a catalyst, it is not desirable to emit this gaseous pollutant into the atmosphere.
Using ammonia for reducing NO2 has also been proposed. In ground equipment, such as a factory, this solution may be satisfactory. However, this method is not feasibly adaptable for ammonia to be carried in vehicles, due to vibration problems typical of diesel exhaust systems and create the risk of a collision with another vehicle or object.
The main reasons why the above-mentioned diesel engine exhaust measures have not progressed compared to gasoline engine exhaust measures is explained below.
In the case of diesel engines, (1) gasoline engines use an air-fuel ratio controlled before and after the ideal combustion ratio of gasoline and air, which is in direct contrast with diesel engines that use light oil for fuel and air is invariably overwhelmingly superfluous; (2) catalysis between solid matter catalyst and other types of substances make it react. Since a large part of the reaction occurs within the pores of a solid matter catalyst, other types have to be in the form of gas or liquid to improve combustion. When compared to the case of diesel engines, the exhaust component is different than gasoline engines in that the exhaust includes a greater amount of solid matter PM that sticks to one another, thereby making it difficult for PM to enter the pores of a solid matter catalyst. Also, the properties and origin of the soot affect its ability to be oxidized. These are the main reasons why exhaust measures have not progressed more rapidly.
In fact, in an experiment by this inventor, to serve as a filter to trap PM, foaming stone grains were formed with a large number of pores with only an adhered coating of base metals as the oxidation catalyst, such as nickel, cobalt, etc. PM burned at about 400 degrees centigrade, which is slightly lower than its spontaneous combustion temperature. However, it didn't reach the temperature that exhaust reaches in the DPF while driving, and likewise combustion of carbon monoxide (CO) and hydrocarbon (HC) hardly progressed.
On the other hand, in another experiment performed with only platinum as the precious metals catalyst, it adhered to the aluminum oxide (Al2O3) carrier intermingled with the foaming stone grains. Even though combustion of CO and HC advanced, the PM did not burn but was accumulated on the filter part.
The purpose of this invention is to provide a diesel particulate filter which enables removal of harmful particulate matter (PM) from the exhaust discharged from a diesel engine and incineration of the PM at the lowest possible emission temperature in a diesel engine particulate filter (DPF), without the use of an electric heater.